negative feedback _ operational amplifiers

8/12/2019 Negative Feedback _ Operational Amplifiers

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Negative feedbackIf we connect the output of an op-amp to its inverting input and apply a voltage signal to thenoninverting input, we find that the output voltage of the op-amp closely follows that inputvoltage (I've neglected to draw in the power supply, +V/-V wires, and ground symbol forsimplicity):

As V in increases, V out will increase in accordance with the differential gain. However, as V outincreases, that output voltage is fed back to the inverting input, thereby acting to decrease thevoltage differential between inputs, which acts to bring the output down. What will happen forany given voltage input is that the op-amp will output a voltage very nearly equal to V in , but justlow enough so that there's enough voltage difference left between V in and the (-) input to be

amplified to generate the output voltage.The circuit will quickly reach a point of stability (known as equilibrium in physics), where theoutput voltage is just the right amount to maintain the right amount of differential, which in turnproduces the right amount of output voltage. Taking the op-amp's output voltage and coupling itto the inverting input is a technique known as negative feedback , and it is the key to having aself-stabilizing system (this is true not only of op-amps, but of any dynamic system in general).This stability gives the op-amp the capacity to work in its linear (active) mode, as opposed tomerely being saturated fully "on" or "off" as it was when used as a comparator, with no feedbackat all.

Because the op-amp's gain is so high, the voltage on the inverting input can be maintained almost

equal to V in . Let's say that our op-amp has a differential voltage gain of 200,000. If V in equals 6volts, the output voltage will be 5.999970000149999 volts. This creates just enough differentialvoltage (6 volts - 5.999970000149999 volts = 29.99985 V) to c ause 5.999970000149999 volts tobe manifested at the output terminal, and the system holds there in balance. As you can see,29.99985 V is not a lot of differential, so for practical calculations, we can assume that thedifferential voltage between the two input wires is held by negative feedback exactly at 0 volts.


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One great advantage to using an op-amp with negative feedback is that the actual voltage gainof the op-amp doesn't matter, so long as its very large. If the op-amp's differential gain were250,000 instead of 200,000, all it would mean is that the output voltage would hold just a littlecloser to V in (less differential voltage needed between inputs to generate the required output). Inthe circuit just illustrated, the output voltage would still be (for all practical purposes) equal tothe non-inverting input voltage. Op-amp gains, therefore, do not have to be precisely set by thefactory in order for the circuit designer to build an amplifier circuit with precise gain. Negativefeedback makes the system self-correcting. The above circuit as a whole will simply follow theinput voltage with a stable gain of 1.

Going back to our differential amplifier model, we can think of the operational amplifier as being avariable voltage source controlled by an extremely sensitive null detector , the kind of metermovement or other sensitive measurement device used in bridge circuits to detect a condition of balance (zero volts). The "potentiometer" inside the op-amp creating the variable voltage willmove to whatever position it must to "balance" the inverting and noninverting input voltages sothat the "null detec tor" has zero voltage ac ross it:

As the "potentiometer" will move to provide an output voltage necessary to satisfy the "nulldetector" at an "indication" of zero volts, the output voltage becomes equal to the input voltage:in this case, 6 volts. If the input voltage changes at all, the "potentiometer" inside the op-amp willchange position to hold the "null detector" in balance (indicating zero volts), resulting in an outputvoltage approximately equal to the input voltage at all times.


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This will hold true within the range of voltages that the op-amp can output. With a power supplyof +15V/-15V, and an ideal amplifier that can swing its output voltage just as far, it will faithfully"follow" the input voltage between the limits of +15 volts and -15 volts. For this reason, theabove circuit is known as a voltage follower . Like its one-transistor counterpart, the common-collector ("emitter-follower") amplifier, it has a voltage gain of 1, a high input impedance, a lowoutput impedance, and a high current gain. Voltage followers are also known as voltage buffers ,and are used to boost the current-sourcing ability of voltage signals too weak (too high of sourceimpedance) to directly drive a load. The op-amp model shown in the last illustration depicts howthe output voltage is essentially isolated from the input voltage, so that current on the output pinis not supplied by the input voltage source at all, but rather from the power supply powering the

op-amp.It should be mentioned that many op-amps cannot swing their output voltages exactly to +V/-Vpower supply rail voltages. The model 741 is one of those that cannot: when saturated, its outputvoltage peaks within about one volt of the +V power supply voltage and within about 2 volts of the -V power supply voltage. Therefore, with a split power supply of +15/-15 volts, a 741 op-amp's output may go as high as +14 volts or as low as -13 volts (approximately), but no further.This is due to its bipolar transistor design. These two voltage limits are known as the positivesaturat ion voltage and negative saturat ion voltage , respect ively. Other op-amps, such as themodel 3130 with field-effect transistors in the final output stage, have the ability to swing theiroutput voltages within millivolts of either power supply rail voltage. Consequently, their positive

and negative saturation voltages are practically equal to the supply voltages.REVIEW:

Connecting the output of an op-amp to its inverting (-) input is called negative feedback .This term can be broadly applied to any dynamic system where the output signal is "fedback" to the input somehow so as to reach a point of equilibrium (balance).When the output of an op-amp is directly connected to its inverting (-) input, a voltagefollower will be created. Whatever signal voltage is impressed upon the noninverting (+)input will be seen on the output.An op-amp with negative feedback will try to drive its output voltage to whatever levelnecessary so t hat t he differential voltage between the two inputs is prac tically zero. The

higher the op-amp differential gain, the closer that differential voltage will be to zero.Some op-amps cannot produce an output voltage equal to their supply voltage whensaturated. The model 741 is one of these. The upper and lower limits of an op-amp'soutput voltage swing are known as positive saturation voltage and negative saturationvoltage , respect ively.

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negative feedback _ operational amplifiers

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